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<h1 class="title">
<a id="glossary"></a>Glossary of terms<a class="edit_me edit_me_private" rel="nofollow" title="Editing on GitHub is available to Elastic" href="https://github.com/elastic/elasticsearch/edit/7.7/docs/reference/glossary.asciidoc">edit</a>
</h1>
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<dl>
<dt>
<span class="glossterm">
<a id="glossary-analysis"></a> analysis
</span>
</dt>
<dd class="glossdef">
<p>
Analysis is the process of converting <a class="xref" href="glossary.html#glossary-text">full text</a> to
<a class="xref" href="glossary.html#glossary-term">terms</a>. Depending on which analyzer is used, these phrases:
<code class="literal">FOO BAR</code>, <code class="literal">Foo-Bar</code>, <code class="literal">foo,bar</code> will probably all result in the
terms <code class="literal">foo</code> and <code class="literal">bar</code>. These terms are what is actually stored in
the index.
</p>
<p>A full text query (not a <a class="xref" href="glossary.html#glossary-term">term</a> query) for <code class="literal">FoO:bAR</code> will
also be analyzed to the terms <code class="literal">foo</code>,<code class="literal">bar</code> and will thus match the
terms stored in the index.</p>
<p>It is this process of analysis (both at index time and at search time)
that allows Elasticsearch to perform full text queries.</p>
<p>Also see <a class="xref" href="glossary.html#glossary-text">text</a> and <a class="xref" href="glossary.html#glossary-term">term</a>.</p>
</dd>
<dt>
<span class="glossterm">
<a id="glossary-cluster"></a> cluster
</span>
</dt>
<dd class="glossdef">
A cluster consists of one or more <a class="xref" href="glossary.html#glossary-node">nodes</a> which share the
same cluster name. Each cluster has a single master node which is
chosen automatically by the cluster and which can be replaced if the
current master node fails.
</dd>
<dt>
<span class="glossterm">
<a id="glossary-ccr"></a> cross-cluster replication (CCR)
</span>
</dt>
<dd class="glossdef">
The cross-cluster replication feature enables you to replicate indices in remote clusters to your
local cluster. For more information, see
<a href="xpack-ccr.html" class="ulink" target="_top">Cross-cluster replication</a>.
</dd>
<dt>
<span class="glossterm">
<a id="glossary-ccs"></a> cross-cluster search (CCS)
</span>
</dt>
<dd class="glossdef">
The cross-cluster search feature enables any node to act as a federated client across
multiple clusters. See <a class="xref" href="modules-cross-cluster-search.html" title="Search across clusters"><em>Search across clusters</em></a>.
</dd>
<dt>
<span class="glossterm">
<a id="glossary-document"></a> document
</span>
</dt>
<dd class="glossdef">
<p>
A document is a JSON document which is stored in Elasticsearch. It is
like a row in a table in a relational database. Each document is
stored in an <a class="xref" href="glossary.html#glossary-index">index</a> and has a <a class="xref" href="glossary.html#glossary-type">type</a> and an
<a class="xref" href="glossary.html#glossary-id">id</a>.
</p>
<p>A document is a JSON object (also known in other languages as a hash /
hashmap / associative array) which contains zero or more
<a class="xref" href="glossary.html#glossary-field">fields</a>, or key-value pairs.</p>
<p>The original JSON document that is indexed will be stored in the
<a class="xref" href="glossary.html#glossary-source_field"><code class="literal">_source</code> field</a>, which is returned by default when
getting or searching for a document.</p>
</dd>
<dt>
<span class="glossterm">
<a id="glossary-field"></a> field
</span>
</dt>
<dd class="glossdef">
<p>
A <a class="xref" href="glossary.html#glossary-document">document</a> contains a list of fields, or key-value
pairs. The value can be a simple (scalar) value (eg a string, integer,
date), or a nested structure like an array or an object. A field is
similar to a column in a table in a relational database.
</p>
<p>The <a class="xref" href="glossary.html#glossary-mapping">mapping</a> for each field has a field <em>type</em> (not to
be confused with document <a class="xref" href="glossary.html#glossary-type">type</a>) which indicates the type
of data that can be stored in that field, eg <code class="literal">integer</code>, <code class="literal">string</code>,
<code class="literal">object</code>. The mapping also allows you to define (amongst other things)
how the value for a field should be analyzed.</p>
</dd>
<dt>
<span class="glossterm">
<a id="glossary-filter"></a> filter
</span>
</dt>
<dd class="glossdef">
A filter is a non-scoring <a class="xref" href="glossary.html#glossary-query">query</a>, meaning that it does not score documents.
It is only concerned about answering the question - "Does this document match?".
The answer is always a simple, binary yes or no. This kind of query is said to be made
in a <a class="xref" href="query-filter-context.html" title="Query and filter context">filter context</a>,
hence it is called a filter. Filters are simple checks for set inclusion or exclusion.
In most cases, the goal of filtering is to reduce the number of documents that have to be examined.
</dd>
<dt>
<span class="glossterm">
<a id="glossary-follower-index"></a> follower index
</span>
</dt>
<dd class="glossdef">
Follower indices are the target indices for <a class="xref" href="glossary.html#glossary-ccr">cross-cluster replication</a>. They exist
in your local cluster and replicate <a class="xref" href="glossary.html#glossary-leader-index">leader indices</a>.
</dd>
<dt>
<span class="glossterm">
<a id="glossary-force-merge"></a> force merge
</span>
</dt>
<dd class="glossdef">
Manually trigger a merge to reduce the number of segments in each shard of an index
and free up the space used by deleted documents.
You should not force merge indices that are actively being written to.
Merging is normally performed automatically, but you can use force merge after
<a class="xref" href="glossary.html#glossary-rollover">rollover</a> to reduce the shards in the old index to a single segment.
See the <a href="indices-forcemerge.html" class="ulink" target="_top">force merge API</a>.
</dd>
<dt>
<span class="glossterm">
<a id="glossary-freeze"></a> freeze
</span>
</dt>
<dd class="glossdef">
Make an index read-only and minimize its memory footprint.
Frozen indices can be searched without incurring the overhead of of re-opening a closed index,
but searches are throttled and might be slower.
You can freeze indices to reduce the overhead of keeping older indices searchable
before you are ready to archive or delete them.
See the <a href="freeze-index-api.html" class="ulink" target="_top">freeze API</a>.
</dd>
<dt>
<span class="glossterm">
<a id="glossary-id"></a> id
</span>
</dt>
<dd class="glossdef">
The ID of a <a class="xref" href="glossary.html#glossary-document">document</a> identifies a document. The
<code class="literal">index/id</code> of a document must be unique. If no ID is provided,
then it will be auto-generated. (also see <a class="xref" href="glossary.html#glossary-routing">routing</a>)
</dd>
<dt>
<span class="glossterm">
<a id="glossary-index"></a> index
</span>
</dt>
<dd class="glossdef">
<p>
An index is like a <em>table</em> in a relational database. It has a
<a class="xref" href="glossary.html#glossary-mapping">mapping</a> which contains a <a class="xref" href="glossary.html#glossary-type">type</a>,
which contains the <a class="xref" href="glossary.html#glossary-field">fields</a> in the index.
</p>
<p>An index is a logical namespace which maps to one or more
<a class="xref" href="glossary.html#glossary-primary-shard">primary shards</a> and can have zero or more
<a class="xref" href="glossary.html#glossary-replica-shard">replica shards</a>.</p>
</dd>
<dt>
<span class="glossterm">
<a id="glossary-index-alias"></a> index alias
</span>
</dt>
<dd class="glossdef">
<p>An index alias is a secondary name
used to refer to one or more existing indices.</p>
<p>Most Elasticsearch APIs accept an index alias
in place of an index name.</p>
<p>See <a href="indices-add-alias.html" class="ulink" target="_top">Add index alias</a>.</p>
</dd>
<dt>
<span class="glossterm">
<a id="glossary-index-template"></a> index template
</span>
</dt>
<dd class="glossdef">
<p>Defines settings and mappings to apply to new indexes that match a simple naming pattern, such as <em>logs-*</em>.
An index template can also attach a lifecycle policy to the new index.
Index templates are used to automatically configure indices created during <a class="xref" href="glossary.html#glossary-rollover">rollover</a>.</p>
</dd>
<dt>
<span class="glossterm">
<a id="glossary-leader-index"></a> leader index
</span>
</dt>
<dd class="glossdef">
Leader indices are the source indices for <a class="xref" href="glossary.html#glossary-ccr">cross-cluster replication</a>. They exist
on remote clusters and are replicated to
<a class="xref" href="glossary.html#glossary-follower-index">follower indices</a>.
</dd>
<dt>
<span class="glossterm">
<a id="glossary-mapping"></a> mapping
</span>
</dt>
<dd class="glossdef">
<p>
A mapping is like a <em>schema definition</em> in a relational database. Each
<a class="xref" href="glossary.html#glossary-index">index</a> has a mapping, which defines a <a class="xref" href="glossary.html#glossary-type">type</a>,
plus a number of index-wide settings.
</p>
<p>A mapping can either be defined explicitly, or it will be generated
automatically when a document is indexed.</p>
</dd>
<dt>
<span class="glossterm">
<a id="glossary-node"></a> node
</span>
</dt>
<dd class="glossdef">
<p>
A node is a running instance of Elasticsearch which belongs to a
<a class="xref" href="glossary.html#glossary-cluster">cluster</a>. Multiple nodes can be started on a single
server for testing purposes, but usually you should have one node per
server.
</p>
<p>At startup, a node will use unicast to discover an existing cluster with
the same cluster name and will try to join that cluster.</p>
</dd>
<dt>
<span class="glossterm">
<a id="glossary-primary-shard"></a> primary shard
</span>
</dt>
<dd class="glossdef">
<p>
Each document is stored in a single primary <a class="xref" href="glossary.html#glossary-shard">shard</a>. When
you index a document, it is indexed first on the primary shard, then
on all <a class="xref" href="glossary.html#glossary-replica-shard">replicas</a> of the primary shard.
</p>
<p>By default, an <a class="xref" href="glossary.html#glossary-index">index</a> has one primary shard. You can specify
more primary shards to scale the number of <a class="xref" href="glossary.html#glossary-document">documents</a>
that your index can handle.</p>
<p>You cannot change the number of primary shards in an index, once the index is
created. However, an index can be split into a new index using the
<a class="xref" href="indices-split-index.html" title="Split index API">split API</a>.</p>
<p>See also <a class="xref" href="glossary.html#glossary-routing">routing</a></p>
</dd>
<dt>
<span class="glossterm">
<a id="glossary-query"></a> query
</span>
</dt>
<dd class="glossdef">
<p>
A request for information from Elasticsearch. You can think of a query as a question,
written in a way Elasticsearch understands. A search consists of one or more queries
combined.
</p>
<p>There are two types of queries: <em>scoring queries</em> and <em>filters</em>. For more
information about query types, see <a class="xref" href="query-filter-context.html" title="Query and filter context"><em>Query and filter context</em></a>.</p>
</dd>
<dt>
<span class="glossterm">
<a id="glossary-recovery"></a> recovery
</span>
</dt>
<dd class="glossdef">
<p>Shard recovery is the process
of syncing a <a class="xref" href="glossary.html#glossary-replica-shard">replica shard</a>
from a <a class="xref" href="glossary.html#glossary-primary-shard">primary shard</a>.
Upon completion,
the replica shard is available for search.</p>
<p>Recovery automatically occurs
during the following processes:</p>
<div class="ulist itemizedlist">
<ul class="itemizedlist">
<li class="listitem">
Node startup or failure.
This type of recovery is called a <span class="strong strong"><strong>local store recovery</strong></span>.
</li>
<li class="listitem">
<a class="xref" href="glossary.html#glossary-replica-shard">Primary shard replication</a>.
</li>
<li class="listitem">
Relocation of a shard to a different node in the same cluster.
</li>
<li class="listitem">
<a href="snapshots-restore-snapshot.html" class="ulink" target="_top">Snapshot restoration</a>.
</li>
</ul>
</div>
</dd>
<dt>
<span class="glossterm">
<a id="glossary-reindex"></a> reindex
</span>
</dt>
<dd class="glossdef">
To cycle through some or all documents in one or more indices, re-writing them into the same or new index in a local or remote cluster. This is most commonly done to update mappings, or to upgrade Elasticsearch between two incompatible index versions.
</dd>
<dt>
<span class="glossterm">
<a id="glossary-replica-shard"></a> replica shard
</span>
</dt>
<dd class="glossdef">
<p>
Each <a class="xref" href="glossary.html#glossary-primary-shard">primary shard</a> can have zero or more
replicas. A replica is a copy of the primary shard, and has two
purposes:
</p>
<div class="olist orderedlist">
<ol class="orderedlist">
<li class="listitem">
increase failover: a replica shard can be promoted to a primary
shard if the primary fails
</li>
<li class="listitem">
<p>increase performance: get and search requests can be handled by
primary or replica shards.</p>
<p>By default, each primary shard has one replica, but the number of
replicas can be changed dynamically on an existing index. A replica
shard will never be started on the same node as its primary shard.</p>
</li>
</ol>
</div>
</dd>
<dt>
<span class="glossterm">
<a id="glossary-rollover"></a> rollover
</span>
</dt>
<dd class="glossdef">
<p>Redirect an alias to begin writing to a new index when the existing index reaches a certain age, number of docs, or size.
The new index is automatically configured according to any matching <a class="xref" href="glossary.html#glossary-index-template">index templates</a>.
For example, if you’re indexing log data, you might use rollover to create daily or weekly indices.
See the <a href="indices-rollover-index.html" class="ulink" target="_top">rollover index API</a>.</p>
</dd>
<dt>
<span class="glossterm">
<a id="glossary-routing"></a> routing
</span>
</dt>
<dd class="glossdef">
<p>
When you index a document, it is stored on a single
<a class="xref" href="glossary.html#glossary-primary-shard">primary shard</a>. That shard is chosen by hashing
the <code class="literal">routing</code> value. By default, the <code class="literal">routing</code> value is derived from
the ID of the document or, if the document has a specified parent
document, from the ID of the parent document (to ensure that child and
parent documents are stored on the same shard).
</p>
<p>This value can be overridden by specifying a <code class="literal">routing</code> value at index
time, or a <a class="xref" href="mapping-routing-field.html" title="_routing field">routing
field</a> in the <a class="xref" href="glossary.html#glossary-mapping">mapping</a>.</p>
</dd>
<dt>
<span class="glossterm">
<a id="glossary-shard"></a> shard
</span>
</dt>
<dd class="glossdef">
<p>A shard is a single Lucene instance. It is a low-level “worker” unit
which is managed automatically by Elasticsearch. An index is a logical
namespace which points to <a class="xref" href="glossary.html#glossary-primary-shard">primary</a> and
<a class="xref" href="glossary.html#glossary-replica-shard">replica</a> shards.</p>
<p>+
Other than defining the number of primary and replica shards that an
index should have, you never need to refer to shards directly.
Instead, your code should deal only with an index.</p>
<p>+
Elasticsearch distributes shards amongst all <a class="xref" href="glossary.html#glossary-node">nodes</a> in the
<a class="xref" href="glossary.html#glossary-cluster">cluster</a>, and can move shards automatically from one
node to another in the case of node failure, or the addition of new
nodes.</p>
</dd>
<dt>
<span class="glossterm">
<a id="glossary-shrink"></a> shrink
</span>
</dt>
<dd class="glossdef">
Reduce the number of primary shards in an index.
You can shrink an index to reduce its overhead when the request volume drops.
For example, you might opt to shrink an index once it is no longer the write index.
See the <a href="indices-shrink-index.html" class="ulink" target="_top">shrink index API</a>.
</dd>
<dt>
<span class="glossterm">
<a id="glossary-source_field"></a> source field
</span>
</dt>
<dd class="glossdef">
By default, the JSON document that you index will be stored in the
<code class="literal">_source</code> field and will be returned by all get and search requests.
This allows you access to the original object directly from search
results, rather than requiring a second step to retrieve the object
from an ID.
</dd>
<dt>
<span class="glossterm">
<a id="glossary-term"></a> term
</span>
</dt>
<dd class="glossdef">
<p>
A term is an exact value that is indexed in Elasticsearch. The terms
<code class="literal">foo</code>, <code class="literal">Foo</code>, <code class="literal">FOO</code> are NOT equivalent. Terms (i.e. exact values) can
be searched for using <em>term</em> queries.
</p>
<p>See also <a class="xref" href="glossary.html#glossary-text">text</a> and <a class="xref" href="glossary.html#glossary-analysis">analysis</a>.</p>
</dd>
<dt>
<span class="glossterm">
<a id="glossary-text"></a> text
</span>
</dt>
<dd class="glossdef">
<p>
Text (or full text) is ordinary unstructured text, such as this
paragraph. By default, text will be <a class="xref" href="glossary.html#glossary-analysis">analyzed</a> into
<a class="xref" href="glossary.html#glossary-term">terms</a>, which is what is actually stored in the index.
</p>
<p>Text <a class="xref" href="glossary.html#glossary-field">fields</a> need to be analyzed at index time in order to
be searchable as full text, and keywords in full text queries must be
analyzed at search time to produce (and search for) the same terms
that were generated at index time.</p>
<p>See also <a class="xref" href="glossary.html#glossary-term">term</a> and <a class="xref" href="glossary.html#glossary-analysis">analysis</a>.</p>
</dd>
<dt>
<span class="glossterm">
<a id="glossary-type"></a> type
</span>
</dt>
<dd class="glossdef">
A type used to represent the <em>type</em> of document, e.g. an <code class="literal">email</code>, a <code class="literal">user</code>, or a <code class="literal">tweet</code>.
Types are deprecated and are in the process of being removed.  See <a class="xref" href="removal-of-types.html" title="Removal of mapping types"><em>Removal of mapping types</em></a>.
</dd>
</dl>
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